Although approximately 90% of Alzheimer’s disease cases occur sporadically, mutated genes coding for either amyloid precursor protein (chromosome 21) or presenilin-1 (PS1; chromosome 14) or presenilin-2 (PS2; chromosome 1) are found in early-onset familial forms of AD [46]. Here, PSEN1 is linked to early-onset autosomal dominant Alzheimer disease.